A material such as a steel sheet is coated with a Zn plating layer and then with paint, in order to improve the corrosion resistance of a building or structrual material in various fields of use. The material after being coated with a Zn plating layer is usually subjected to phosphating treatment before paint application. A film composed of fine phosphate crystals is formed on the surface of the substrate material by the phosphating treatment. The phosphate film effectively improves the adhesiveness of a paint film to the surface of the substrate steel, so that the obtained product is excellent in corrosion resistance and weatherability.
Corrosion resistance required for building and structural materials has become more severe, as the environment is getting worse in corrosive condition. In this regards, a material coated with a Zn or Zn alloy plating layer does not exhibit sufficient corrosion resistance, so that corrosion is often observed under the paint film. In addition, a brittle alloyed layer is likely to be formed between the plating layer and the steel substrate. The brittle alloyed layer would cause the formation of various defects such as flaking and powdering in the plating layer, when the coated steel sheet is worked to an objective shape.
In order to acquire enough endurance against the severer environment for use, there have been proposed various improvements such as the selection of substrate steel, the modification of the plating layer in composition and the change of a plating method to improve corrosion resistance. For instance, a stainless steel is used as a substrate to be coated with a Zn plating layer. The Zn-coated stainless steel exhibits extremely excellent corrosion resistance owing to the co-operative effect of the stainless steel with the Zn plating layer.
Vapor depostion plating has been noted in recent years as a substitution for conventional hot-dip or electrolytic plating. According to the vapor deposition, it is possible to obtain a plating layer having a composition, which can not be formed by the conventional method, with the high degree of freedom. For instance, Japanese Patent Application Laid-Open 63-247353 discloses a vapor deposition plating method for forming a Zn-Al alloy plating layer on the surface of a steel sheet. In this method, Zn and Al are independently evaporated from different vessels, and Zn and Al are simultaneously deposited from the vapor phase on the surface of the steel sheet.
The vapor deposition plating layer adheres onto the surface of the steel substrate without substantial reaction between the plating layer and the substrate steel, so that the coated steel sheet exhibits excellent workability as compared with a Zn alloy-coated steel sheet manufactured by a hot-dip plating method or the like. In addition, the coated steel sheet is excellent in corrosion resistance, too, due to Al contained in the plating layer.
However, during the formation of a vapor deposition plating layer, there is a possibility that the plating layer would be formed in a state which has poor adhesiveness and defects such as pores would be involved in response to the surface conditions of the material such as a steel sheet to be coated. In order to improve the adhesiveness of the vapor depostion plating layer, there has been proposed a pretreatment for activating the surface of the steel sheet by electron beam heating, gas reduction or the like before the vacuum deposition of a plating metal on the surface of the steel sheet.
When the surface of the steel sheet is activated by electron beam heating, it is necessary to elevate the surface temperature of the steel sheet at the begining of vapor deposition. For instance, in the case where Ti and Al are to be applied as a first sub-layer and a second sub-layer, respectively, onto the surface of the steel sheet, the surface of the steel sheet shall be heated at a temperature of 400.degree. C. or higher to improve the adhesiveness of the multi-layered plating layer.
However, the heating promotes the diffusion of Al into the substrate steel, so that a brittle intermetallic compound is likely to be formed. Consequently, when the coated steel sheet is worked to an objective shape, there would appear in the plating layer various defects such as peeling, powdering and flaking, just like a coated steel sheet manufactured by the hot-dip coating method. In case where a material such as Zn having a lower volatile point is used as a plating metal, the plating metal is easily re-evaporated from the surface of the steel sheet due to the heat of the activation during vapor deposition. Consequently, it is difficult to obtain a plating layer having the desired composition. In order to avoid such defects, strict temperature control is required to hold the surface of the steel sheet at a temperature below 500.degree. C.
On the other hand, the gas reduction method for activating the surface of the steel sheet requires a large-scale gas reducing furnace and various supplemental equipment. Due to the large scale of the furnace, temperature control in the furnace and sealing mechanism become troublesome, and it takes a long time to raise or lower the temperature of the furnace. Consequently, productivity is lowered.
In a conventional paint applying process line, various steps are combined with each other mainly for applying paint to a Zn-coated steel sheet. When a steel material coated with an Al-Zn alloy plating layer containing a large amount of Al is introduced into this paint applying process line, a phosphate film is not uniformly formed on the surface of the steel material, and a paint film formed thereon lacks homogeneity. Besides, the surface of the steel material would be exposed to the atmosphere at its dent or corner part without the adhesion of paint, especially when the steel material has complicated configuration.
In order to overcome the aforesaid problems, we have proposed a new method whereby the surface of a steel sheet is activated by radiation with ion beams and then subjected to vapor deposition plating, as disclosed in Japanese Patent Application Laid-Open 64-56862. The irradiation with ion beams utilizes the etching reaction derived from the bombardments of accelerated ions beams into the surface of the steel sheet, different from the known method wherein the surface of the steel sheet is thermally activated by electron beam heating. The surface of the steel sheet irradiated with ion beams is converted into a surely activated state suitable as a substrate for a vapor deposition plating layer excellent in adhesiveness.
We have researched and examined the ion beam irradiation from various points of view to further enhance the activation effect. In the course of the research and examination, we have found that the surface temperature of the steel sheet at the beginning of vapor deposition has a big influence on the maintenance of the activated state.
An object of the present invention is to improve the adhesiveness, density and corrosion resistance of a vapor deposition plating layer formed on the surface of a steel material.
Another object of the present invention is to use the activated state of the steel material effectively for vapor deposition plating by maintaining the surface of the steel material at a temperature within the range of 100.degree.-400.degree. C. at the begining of vapor deposition.